It is the longest sentence meted out so far over the probe involving billions of dollars allegedly misappropriated from Malaysian state fund 1Malaysia Development Berhad (1MDB).

Yeo, 34, is already in jail. He was sentenced to 2½ years' jail last December after being convicted of four charges of obstruction of justice. The new sentence started yesterday and runs concurrently with the rest of that jail term.

Yeo pleaded guilty yesterday to one charge each of money laundering and cheating, with eight other charges taken into consideration.

Prosecutors said Yeo played a central role in transactions involving sovereign wealth funds SRC International and 1MDB, and 1MDB unit Brazen Sky, and made secret profits on the side. At an earlier hearing, they said Yeo amassed $23.9 million in "secret profits" in 15 months after he left BSI in June 2014.

Prosecutors said he worked for Malaysian businessman Low Taek Jho, who remains a "key person of interest" in a wider ongoing money-laundering probe.

The two charges relate to a series of transactions involving Brazen Sky apparently aimed at covering up the transfer of US$1 billion (S$1.4 billion) belonging to 1MDB to a bank account Mr Low beneficially owned.

Prosecutor Nathaniel Khng said that investigations showed Mr Low was a "key adviser" to 1MDB and SRC, and that he received huge sums of money traceable to 1MDB for his own benefit, his family's and that of his close associates.

Yeo and his superior, Mr Kevin Swampillai, former BSI head of wealth management services, had put in place plans to obtain secret profits from fees paid to fiduciary funds involved in the transactions.

The schemes were concealed from BSI Singapore and resulted in Yeo earning more than US$3.5 million in illicit profits.

He then laundered part of the proceeds into Singapore. One charge yesterday related to Yeo's laundering of US$500,000.

In his mitigation plea yesterday, Yeo said he regarded the transactions as "profitable opportunities with an important client" and was remorseful.

He also agreed to surrender to the Singapore authorities a substantial part of the $23.9 million he had amassed and to help with the ongoing probe.

Principal District Judge Ong Hian Sun said the courts must take an "uncompromising stance" to safeguard the integrity of the financial system here.

"Yeo had exploited his position for his own benefit out of pure greed," he said.